FIG. 1 illustrates a flat plate automotive exhaust sensor of the prior art. The exhaust sensor includes a heated flat plate sensing element 138 which is carried in a tubular housing 116 and held in position by a first cement composition 140, glass seal 142 and a second cement composition 144, each of which extends from the flat plate heating element to the tubular wall 116 of the sensor. The heated flat plate sensing element includes an air reference channel formed therein and is positioned within the sensor to provide communication with an air source in the upper portion of the sensor. A glass seal prevents exhaust gas from travelling from the lower end of the sensor through the tubular housing and into the upper portion of the sensor to contaminate the air in the air reference channel of the sensing element. Glass seals are desirable because they are easily formed by firing glass frit in a furnace. Such sensors are used to monitor constituents in an automotive combustion engine exhaust gas stream such as oxygen, and to adjust the operation of the engine including the air/fuel ratio.
However, the glass seal, heated flat plate sensing element and the tubular shell have different thermal coefficients of expansion. When the sensor is exposed to high temperatures associated with combustion engine exhaust, the glass seal, heated flat plate sensing element and the tubular shell expand and contract at different rates. This often results in leakage paths between the glass seal and the tubular shell, thereby contaminating the air reference with exhaust gases.
The present invention overcomes many of the deficiencies of the prior art sensors.